Conventional InGaAsP/InP laser structures, including the planar buried heterostructure and the buried crescent heterostructure, consist of a thin InGaAsP active layer sandwiched between InP cladding layers on a thick InP substrate. In these structures, the internal stress, caused by lattice mismatch and the differential thermal expansion, exists mostly in the thin InGaAsP active layer. The stress is larger for lasers operating at longer wavelengths because of a larger compositional mismatch between the active layer and the surrounding InP layers, which results in a larger difference in the thermal expansion coefficients. Under usual liquid phase epitaxial (LPE) growth conditions, this stress tends to be tensile along the junction plane (.sigma..sub.xx &gt;0) at room temperature.
In U.S. patent application entitled "Fast Polarization-Switchable Semiconductor Lasers", Ser. No. 683,776 by Liu and Chen, filed concurrently herewith, a net tensile stress on the order of 10.sup.8 dyn/cm.sup.2 is created in the active layer of a semiconductor laser to induce sufficient lattice deformation to promote a TM mode gain large enough to compete with the normal operating TE mode which allows the polarization of the laser output to be switched by varying injection current level. In various applications for the InGaAsP laser, the existence of net tensile stress in the active layer produces undesirable characteristics. For example, an InGaAsP/InP laser may change polarization with environmental temperature changes due to thermal stress in the active layer. In fact, InGaAsP/InP lasers are observed to operate in a mixture of TE and TM modes at room temperature under normal operating conditions. Furthermore, at high injection currents, kinks in the power-current characteristics associated with the appearance of higher order TM modes are observed in some InGaAsP/InP lasers. These kinks are caused by a combination of several effects. However, the stress in the active layer intensifies the problem. This problem is very important for InGaAsP/InP lasers operating in the wavelength range from 1.3 .mu.m to 1.55 .mu.m for optical communications. These problems do not exist in conventional AlGaAs/GaAs lasers with thin active layers because of their structural differences. The thermal stress in the active layer of an AlGaAs/GaAs laser induced by the thin cladding layers is compressive in the wavelength range from 8000 to 8500 Angstroms. Such compressive stress enhances the gain of the TE mode. In the longer wavelength range, the stress becomes tensile. However, the stress in the active layer of an AlGaAs/GaAs laser is always largely offset by the thick GaAs substrate whose composition is similar to that of the active layer.